This invention relates to refrigerant compressor motor protection and, more particularly, relates to apparatus for more reliably protecting the electric drive motor of a refrigerant compressor.
A variety of devices have been produced for the purpose of protecting three-phase AC motors. Some of these devices include the provision of first and second switches arranged so that the first switch will open to disconnect the motor from its voltage source in the event that the second switch fails in the closed position.
One such device is illustrated in U.S. Pat. No. 2,470,257 (Moore -- May 17, 1949), which illustrates a protection circuit in which a main or working contactor normally controls the flow of electrical current to a three-phase motor. A safety switch is normally closed and is opened only if the working contactor fails in the closed position. The present invention provides an improvement over such arrangement. A normally closed safety switch has a tendency to stick in the closed position due to wear and contamination. Contamination may result from different sources, for example, insect juices, which are sticky and glue-like. As a result, after a period of time in field service, the safety switch may become ineffective and useless as a safety switch.
In order to overcome the deficiencies of the prior art, it has been suggested that the compressor motor can be protected by two contactors connected in series between the motor and a source of AC voltage, which contactors are both normally opened and closed in response to a demand for operation of the motor. By periodically opening and closing both of the contactors, the overall arrangement is less susceptible to failure in field service. See, the copending application of Richard E. Cawley, Ser. No. 483,293 filed June 26, 1974, now U.S. Pat. No. 3,898,527. The present invention provides another solution utilizing two contactors which enhances protection reliability.
A principal object of the present invention is to provide an improved compressor motor protection apparatus embodying first and second contactors connected between a compressor motor and an AC source, which contactors are normally both opened and closed in response to demand for operation of the compressor motor, and including auxiliary high impedance relay means for preventing operation of the compressor motor in the event one contactor fails closed.
It is another object of the present invention to provide an improved circuit arrangement of the foregoing type in which the coil of the high impedance relay means is energized in response to failure of one contactor to open a normally closed contact in a control circuit and thereby, prevent operation of the second contactor and possible damage to the compressor motor.
A circuit protection arrangement of the foregoing type offers a number of advantages, including magnification of system reliability. For example, if the first contactor is 99.5 percent reliable from the standpoint of failing in the closed position, the odds for causing a compressor failure by using the first contactor alone would be 0.5 percent or 1 in 200. However, if a 99.5 percent reliable second contactor is operatively connected for simultaneous operation with the first contactor, the odds of a compressor failure are greatly reduced, since both switches would have to fail closed. The likelihood of a failure under these circumstances would be ##EQU1## In other words, if the first and second contactors are both 99.5 percent reliable from the standpoint of failing closed, the use of two such contactors improves the system reliability by a factor of 200.
Using first and second contactors operable simultaneously also virtually eliminates failures resulting from specific problems associated with a particular contactor manufacturer. By using two contactors, one of the contactors can be supplied from one manufacturer and the other contactor from another manufacturer. As a result, if one of the manufacturers has a problem associated with its contactor, for example, contact or switch sticking, whereas the other manufacturer does not, the use of both contactors and the auxiliary high impedance relay means would prevent compressor failure.
These and other object and advantages of the present invention will be made more apparent in the specification hereinafter.